The present invention pertains to the art of detecting and controlling the intensity x-ray fluence. It finds particular application in conjunction with annular x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with the generation of radiation for other applications.
Typically, a patient is positioned in a supine position on a horizontal couch through a central bore of a CT scanner. An x-ray tube is mounted on a rotatable gantry portion and rotated around the patient at a high rate of speed. For faster scans, the x-ray tube is rotated more rapidly. However, rotating the x-ray more rapidly decreases the net radiation per image. As CT scanners have become faster, larger x-ray tubes have been developed which generate more radiation per unit time to maintain the desired radiation dose at higher speeds. Larger tubes, of course, cause high inertial forces.
High performance x-ray tubes for CT scanners and the like commonly include a stationary cathode and a rotating anode disk, both enclosed within an evacuated housing. As more intense x-ray beams are generated, there is more heating of the anode disk. In order to provide sufficient time for the anode disk to cool by radiating heat through the vacuum to surrounding fluids, x-ray tubes with progressively larger anode disks have been built.
The larger anode disk requires a larger x-ray tube which does not readily fit in the small confined space of an existing CT scanner gantry. Particularly in a fourth generation scanner, incorporating a larger x-ray tube and heavier duty support structure requires moving the radiation detectors to a larger diameter. A longer radiation path between the x-ray tube and the detectors would require that the detectors would be physically larger to subtend the required solid angle. Larger detectors would be more expensive. Not only is a larger x-ray tube required, larger heat exchange structures are required to remove the larger amount of heat which is generated.
In an x-ray tube with a fixed anode and a fixed cathode or with a rotating anode and fixed cathode, accurate control of the radiation dose delivered to the patient was relatively simple. Because the quality of the reconstructed image is dependent upon the number of x-ray photons that are captured by the radiation detector, accurate control of the number of x-ray photons generated is important for good CT images. In order to control the amount of radiation generated, the x-ray tube current is typically monitored and the cathode current is controlled accordingly. Typically, the cathode is fixed at a high negative voltage relative to the housing and the anode similarly fixed to a high positive voltage. Control of the tube current passing between the anode and the cathode (actually an electron flow cathode to anode) is typically done by controlling the temperature of a directly heated cathode filament. More specifically, the electron density from the filament surface is a function of the temperature, the applied anode/cathode voltage, the geometry of the cathode structure, and the distance from the cathode to the anode. The filament temperature is traditionally controlled by controlling a filament heating current.
Rather than rotating a single x-ray tube around the subject, others have proposed using a switchable array of x-ray tubes, e.g. five or six x-ray tubes in a ring around the subject. See, for example, U.S. Pat. No. 4,274,005 to Yamamura. However, unless the tubes rotate only limited data is generated and only limited image resolution is achieved. If multiple x-ray tubes are rotated, similar mechanical problems are encountered trying to move all the tubes quickly and remove all of the heat.
Still others have proposed constructing an essentially bell-shaped, evacuated x-ray tube envelope with a mouth that is sufficiently large that the patient can be received a limited distance in the well of the tube. See, for example, U.S. Pat. No. 4,122,346 issued Oct. 24, 1978 to Enge or U.S. Pat. No. 4,135,095 issued Jan. 16, 1979 to Watanabe. An x-ray beam source is disposed at the apex of the bell to generate an electron beam which impinges on an anode ring at the mouth to the bell. Electronics are provided for scanning the x-ray beam around the evacuated bell-shaped envelope. One problem with this design is that it is only capable of scanning about 270.degree..
Still others have proposed open bore x-ray tubes. See, for example, U.S. Pat. No. 5,125,012 issued Jun. 23, 1992 to Schittenhelm and U.S. Pat. No. 5,179,583 issued Jan. 12, 1993 to Oikawa. These large diameter tubes are constructed analogous to conventional x-ray tubes with a glass housing and a sealed vacuum chamber. Such tubes are expensive to fabricate and are expensive to repair or rebuild in case of tube failure.
One problem with rotating cathode x-ray tubes resides in the difficulty of monitoring the x-ray tube current. The tube current is generally more easily measured at the anode portion of the tube. Measuring the x-ray tube current with an end grounded anode is difficult. Measuring the x-ray tube filament current on the rotating cathode side within the vacuum is also difficult because the filament is rotating and there is no direct means of measurement.
Another problem is controlling the cathode filament temperature accurately. The filament current is supplied to the rotating cathode by a transformer, capacitive coupling, or the like across a vacuum. Any wobble or variation in the gap between transformer or capacitive elements tends to vary the x-ray tube current. This causes difficulties in controlling the filament current in the rotating cathode assembly.
The present invention contemplates a new and improved toroidal x-ray tube and toroidal x-ray tube CT scanner which provides for improved x-ray photon intensity measurement and control.